Field of the Invention
The present invention relates to a dynamic microphone having an improved air chamber formed in the back portion of a dynamic microphone unit and a method of forming a back-side air chamber.
Description of the Related Art
It is well-known that the omnidirectional dynamic microphone operates by resistance control and the unidirectional dynamic microphone operates by mass control and resistance control.
The resistance control of the dynamic microphone is carried out by an acoustic resistance provided very close to the back side of a diaphragm and a back-side air chamber, in which no acoustic wave enters, provided in the back side of the acoustic resistance. The acoustic resistance is located at the inlet of the back-side air chamber. When the volume of the back-side air chamber is significantly large, the total impedance of the acoustic resistance and the impedance of the air chamber is substantially equivalent to the acoustic resistance.
When the volume of the back-side air chamber is small, the impedance of the air chamber due to the stiffness of the air chamber works in series with the acoustic resistance at low frequency.
Thus for a back-side air chamber designed small, the frequency response at low frequency degrades and the directionality at low frequency also changes. It is unrealistic to infinitely increase the volume of the back-side air chamber. Handheld dynamic microphones, in particular, have more strict limitation on the volume of the back-side air chamber.
When a portion of the wall surrounding the back-side air chamber vibrates, the change in the volume of the air chamber generates acoustic waves, and the acoustic waves reaches the diaphragm via the acoustic resistance. Thus when a portion of the wall of the air chamber vibrates, the directional frequency response of the microphone degrades at and near the frequency of the resonance frequency of the vibration portion of the wall.
When an air chamber having a form of a bottomed pipe is used, a standing wave is generated along the longitudinal direction. The standing wave is generated by the acoustic waves entering the air chamber via the acoustic resistance. Typically, the generation of standing waves is avoided by providing a sponge or the like, which has small acoustic resistance, inside the air chamber.
In this case, the space in which the acoustic resistance provided inside the air chamber to avoid standing waves should be as small as possible, because the acoustic resistance is provided equivalently in series with the acoustic resistance for carrying out the resistance control.
JP 3882268 B1 discloses a dynamic microphone configured to have an open cross sectional area of the back-side air chamber becoming smaller in a cross section further remote from the diaphragm.
In the dynamic microphone disclosed in JP 3882268 B1, the microphone grip (grip case) and the back-side air chamber contained in the microphone grip are formed to have an open cross sectional area decreasing toward the rear end, so as to suppress the generation of standing waves inside the microphone grip and the back-side air chamber.
For the dynamic microphone disclosed in JP 3882268 B1, the external shape of the wall of the back-side air chamber and the internal shape of the microphone grip should be substantially identical to efficiently make use of the volume inside the microphone grip as illustrated in FIG. 1 in JP 3882268 B1.
As explained above, the volume of the back-side air chamber is preferably provided as large as possible. However, the dynamic microphone disclosed in JP 3882268 B1 is disadvantageous in that the volume of the air chamber is limited because the back-side air chamber is formed, for example, in a conical shape.